1. Technical Field
The present invention relates to a solid state imaging system and its driving method.
2. Related Art
As a solid state imaging system to be loaded on mobile phones, digital still cameras and the like, there is a CCD (charge coupled device) image sensor (hereinafter, simply referred to as a “CCD sensor”) and a CMOS image sensor (hereinafter, simply referred to as a “CMOS sensor”).
In recent years, a MOS imaging system depending on a threshold voltage shift (hereinafter, simply referred to as a “substrate modulation sensor”) that provides high picture quality and low power consumption is proposed. Japanese Patent Application Publication 2002-134729 proposes an example of a substrate modulation sensor.
A CCD sensor consumes a large amount of power because the driving voltage is high. Meanwhile, a CCD sensor achieves a CDS (correlated double sampling) feature for noise reduction and a so-called synchronous electronic shutter feature for imaging a fast moving object without distortion. The synchronous electronic shutter feature avoids distortion in an image of an object by simultaneously accumulating the photogenerated charge of a plurality of photoreceiving elements that is aligned two-dimensionally. Therefore, a CCD sensor has generally an advantage of providing high picture quality.
Meanwhile, among CMOS sensors, a CMOS-APS (Active Pixel Sensor) sensor composed of four transistors achieves a CDS feature, although without a synchronous electronic shutter feature. A CMOS sensor has an advantage of low power consumption and low process cost because the driving voltage is generally low. In a general CMOS-APS sensor, a synchronous electronic shutter feature can not be achieved because, on each of the readout lines, a floating diffusion (hereinafter, simply referred to as an “FD”), which is a charge conservation area, must be reset first to read out a noise component and then to read out a signal component, so as to achieve a CDS feature.
Specifically, in a CMOS-APS sensor, for achieving a CDS feature, transistors for transferring charge are sequentially reset on each of the selected lines for reading out pixel signals, first to read out a noise component and then to read out a signal component. Signal components are sequentially read out while being reset on each of the selected lines. Therefore, in the imaging of a fast moving object, there occur gradual gaps in the readout times from the first readout line through the last readout line, causing distortion in a resulting image of an object.
Although it is possible to achieve a synchronous electronic shutter in a CMOS-APS sensor, the above-referenced transistors for transferring charge are used to achieve a synchronous electronic shutter feature. Therefore, the achieving of a synchronous electronic shutter feature in a CMOS-APS sensor precludes the achieving of a CDS feature, causing a problem of picture quality degradation.
Further, in a substrate modulation sensor disclosed in the above-referenced Japanese Patent Application Publication 2002-134729, a signal component is first read out and then a noise component is read out after a reset, the difference between the two signal components being output as a pixel signal.
In a substrate modulation sensor, the readout signal component includes a noise component that remained after the last reset, and a noise component to be read out subsequently is a noise component that remained after the reset. The amount of the noise component that remained after the last reset and is included in the signal component is not necessarily equal to the amount of the noise component that remained after the subsequent reset. Specifically, an output pixel signal includes a previous noise component but not a concurrent noise component. Therefore, in a substrate modulation sensor, there is no correlation between a signal component and a noise component, causing a shortcoming that noise is not filtered out precisely, which leads to picture quality degradation.
A technology to achieve a synchronous electronic shutter also in a substrate modulation sensor is proposed, for example, in Japanese Patent Application Publication 2004-87963. In a technology according to the proposal, all the pixels are synchronously reset and then pixel signals are read out sequentially on each of the lines.
Further, in a substrate modulation sensor according to the proposal, a ring gate is used on a modulation transistor. Each of the plural sensor cells that are aligned on a substrate in a two-dimensional matrix has one ring gate for one photodiode.
However, in a technology that is described in the above-referenced Japanese Patent Application Publication 2004-87963, the problem that noise is not filtered out precisely still remains because there is no correlation between a signal component and a noise component as, in the reading out of a pixel signal, a signal component is first read out and then a noise component is read out after a reset.
Further, in a substrate modulation sensor according to the above-referenced two proposals, a certain amount of interval is needed between a ring gate and a drain on a modulation transistor. Therefore, as for a substrate modulation sensor, there is a problem that a substrate modulation sensor can not be downsized because a ring gate needs a certain amount of width.